Image sensors are used in a variety of electronic devices, such as digital cameras, cellular phones, copiers, medical imaging devices, security systems, and time-of-flight cameras. An image sensor typically includes an array of photodetectors that detect or respond to incident light. One type of photodetector that can be used in an image sensor is a single-photon avalanche diode (SPAD) region. An SPAD region is a photosensitive region that is configured to detect low levels of light (down to a single photon) and to signal the arrival times of the photons.
Monolithically-integrated SPAD image sensors typically include an array of SPAD regions and electrical circuitry for the SPAD regions. However, the fill factor of the array can be limited because the electrical circuitry for the SPAD regions consumes space on the semiconductor wafer. Additionally, it can be difficult to prevent contamination of the semiconductor wafer during fabrication of the monolithically-integrated SPAD image sensor. Metals and other contaminants may adversely impact the performance of the SPAD image sensor, such as by increasing noise in the SPAD image sensor.
In some instances, there can be a trade-off between the photon detection efficiency and the timing response of the SPAD regions. A thicker semiconductor wafer can improve the photon detection efficiency of the SPAD regions, but a thicker semiconductor wafer may reduce the timing resolution or response time of the SPAD regions because the charge carriers must propagate through the thicker semiconductor wafer. Additionally, a thicker semiconductor wafer can cause a higher breakdown voltage, which increases the power consumption of the SPAD image sensor when the SPAD image sensor is operating in Geiger mode.